Light-emitting element, light emitting device, display device, electronic appliance, and lighting device

ABSTRACT

A multicolor light-emitting element in which light-emitting layers emitting light of different colors are stacked and color adjustment is easily made is provided. A multicolor light-emitting element which is inexpensive and has favorable emission efficiency is provided. A light-emitting element in which at least two light-emitting layers emitting light of different colors are formed in contact with each other and the light emitted from the two light-emitting layers is obtained from exciplexes is provided. In addition, the light-emitting element in which the exciplexes emit delayed fluorescence is provided.

This application is a continuation of copending U.S. application Ser.No. 14/972,799, filed on Dec. 17, 2015 which is a continuation of U.S.application Ser. No. 13/955,813, filed on Jul. 31, 2013 (now U.S. Pat.No. 9,219,243 issued Dec. 22, 2015), which are all incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting element, a displaydevice, a light-emitting device, an electronic appliance, and a lightingdevice each of which uses an organic compound as a light-emittingsubstance.

2. Description of the Related Art

In recent years, research and development of a light-emitting element(organic EL element) which uses an organic compound and utilizeselectroluminescence (EL) have been actively promoted. In the basicstructure of such a light-emitting element, an organic compound layercontaining a light-emitting substance (an EL layer) is interposedbetween a pair of electrodes. By voltage application to this element,light emission from the light-emitting substance can be obtained.

Such a light-emitting element is a self-luminous element and hasadvantages over a liquid crystal display in having high pixel visibilityand eliminating the need for backlights, for example; thus, such alight-emitting element is thought to be suitable for a flat paneldisplay element. A display including such a light-emitting element isalso highly advantageous in that it can be thin and lightweight.Besides, very high speed response is one of the features of such anelement.

In such a light-emitting element, light-emitting layers can besuccessively formed two-dimensionally, so that planar light emission canbe obtained. Thus, a large-area element can be easily formed. Thisfeature is difficult to obtain with point light sources typified byincandescent lamps and LEDs or linear light sources typified byfluorescent lamps. Thus, the light-emitting element also has greatpotential as a planar light source which can be applied to a lightingdevice and the like.

It is important to obtain white light so that the light-emitting elementis used for lighting. In general, white light can be obtained with theuse of a multicolor light-emitting element which emits light obtained bycombining light from a plurality of emission center substances havingdifferent emission spectra.

Disclosed in Patent Document 1 is a structure in which a layer for coloradjustment is additionally provided in a light-emitting element in whicha plurality of light-emitting layers is stacked. However, this structureincreases the number of constituent elements, and thus isdisadvantageous in terms of cost.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2010-033780

SUMMARY OF THE INVENTION

In the above-described multicolor light-emitting element, obtainingemission of light with different wavelengths at the same time has thesame meaning as obtaining emission of light from different energy levelsat the same time. However, there is inevitably a difference in level inthe emission of light from different energy levels, and as a result,energy transfer might occur.

For this reason, such an element needs to be precisely designed in viewof energy transfer so that a desired emission color is obtained.However, such design takes time and effort.

In view of the above, an object of one embodiment of the presentinvention is to provide a multicolor light-emitting element in whichlight-emitting layers emitting light of different colors are stacked, inwhich color adjustment is easily made.

Another object of one embodiment of the present invention is to providea multicolor light-emitting element which is inexpensive and hasfavorable emission efficiency.

Another object of one embodiment of the present invention is to providea multicolor light-emitting element in which color adjustment is easilymade and which is inexpensive and has favorable emission efficiency.

Another object of one embodiment of the present invention is to providea light-emitting device, a display device, an electronic appliance, anda lighting device each of which can be manufactured at low cost by usingany of the above-described light-emitting elements.

Another object of one embodiment of the present invention is to providea light-emitting device, a display device, an electronic appliance, anda lighting device each of which has reduced power consumption by usingany of the above light-emitting elements.

It is only necessary that at least one of the above-described objects beachieved in the present invention.

In a light-emitting element in which at least two light-emitting layersemitting light of different colors are formed in contact with eachother, light emission of each of the light-emitting layers is obtainedfrom an exciplex, so that the above-described objects can be achieved.

One embodiment of the present invention is a light-emitting elementwhich includes a first electrode, a second electrode, and an EL layerinterposed between the first electrode and the second electrode. The ELlayer includes at least a light-emitting layer in which a firstlight-emitting layer and a second light-emitting layer are stacked. Thefirst light-emitting layer contains a first organic compound and asecond organic compound. The second light-emitting layer contains athird organic compound and a fourth organic compound. The combination ofthe first organic compound and the second organic compound forms a firstexciplex. The combination of the third organic compound and the fourthorganic compound forms a second exciplex.

In a normal light-emitting element, in which light-emitting substanceswhich are not an exciplex are used, energy transfer occurs between thelight-emitting substances, between host materials, or between thelight-emitting substance and the host material because of a differencein band gaps or triplet excited levels. For this reason, adjustment ofthe light-emitting element, such as adjustment of an element structureor doping concentration, which is for obtaining light emission from aplurality of light-emitting layers, becomes complicated. In contrast,energy transfer between exciplexes is less likely to occur; thus, in alight-emitting element having the above-described structure, lightemission can be obtained from two light-emitting layers withoutdifficulty.

The singlet excited level and the triplet excited level of an exciplexare close to each other; thus, reverse intersystem crossing from thetriplet excited state to the singlet excited state easily occurs, anddelayed fluorescence is easily exhibited. Since the delayed fluorescecan convert the triplet excited level to fluorescence, emissionefficiency of a light-emitting can be increased. The difference betweenthe singlet excited state and the triplet excited state is preferablyless than or equal to 0.2 eV, more preferably less than or equal to 0.1eV so that delayed fluorescence is efficiently exhibited.

In view of the above, another embodiment of the present invention is alight-emitting element having the above-described structure, in whichthe first exciplex exhibits delayed fluorescence.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure, in which the second exciplexexhibits delayed fluorescence.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure, in which both of the firstexciplex and the second exciplex exhibit delayed fluorescence.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure, which has an external quantumefficiency of 5% or higher.

Further, in any of the above-described light-emitting elements, arecombination region is formed at an interface between thelight-emitting layers, so that both of the light-emitting layers canefficiently emit light. Further, one of the two substances forming anexciplex has an electron-transport property and the other substance hasa hole-transport property, which is advantageous for formation of theexciplex. Moreover, in such a case, the transport properties of thelight-emitting layers can be easily adjusted depending on the mixtureratio between the two substances, and the recombination region can beeasily adjusted.

Thus, another embodiment of the present invention is a light-emittingelement having the above-described structure, in which an electron-holerecombination region in the light-emitting layer is at an interfacebetween the first light-emitting layer and the second light-emittinglayer.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure, in which one of the first organiccompound and the second organic compound is a substance having anelectron-transport property and the other is a substance having ahole-transport property, and one of the third organic compound and thefourth organic compound is a substance having an electron-transportproperty and the other is a substance having a hole-transport property.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure, in which one of the firstelectrode and the second electrode functions as an anode and the otherfunctions as a cathode, one of the first light-emitting layer and thesecond light-emitting layer, which is positioned on the side where theelectrode functioning as the anode is formed, contains a large amount ofsubstance having a hole-transport property and the other, which ispositioned on the side where the electrode functioning as the cathode isformed, contains a large amount of substance having anelectron-transport property.

Note that in any of the above-described light-emitting elements, theexciplexes emit light with different wavelengths, so that multicolorlight can be obtained by a combination of colors of light emitted fromthe exciplexes. When the exciplexes emit light of complementary colors,white light can be obtained.

Thus, another embodiment of the present invention is a light-emittingelement having the above-described structure, in which the firstexciplex and the second exciplex emit light having peaks at differentwavelengths.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure, in which an emission spectrum hasat least two peaks.

Another embodiment of the present invention is a light-emitting elementhaving the above-described structure, which exhibits white lightemission.

The emission wavelength of the exciplex can be changed by changing oneof the two substances forming the exciplex. In other words, one of thetwo substances forming an exciplex can be common in a plurality oflight-emitting layers. The number of constituent elements is reduced, sothat the element can be manufactured more easily at lower cost.

Thus, another embodiment of the present invention is a light-emittingelement having the above-described structure, in which one of the firstorganic compound and the second organic compound is the same as one ofthe third organic compound and the fourth organic compound.

Another embodiment of the present invention is a light-emitting modulewhich includes the light-emitting element having any of theabove-described structures and a means which controls the light-emittingelement.

Another embodiment of the present invention is a display module whichincludes the light-emitting element having any of the above-describedstructures in a display portion and a means which controls thelight-emitting element.

Another embodiment of the present invention is a lighting device whichincludes the light-emitting element having any of the above-describedstructures.

Another embodiment of the present invention is a light-emitting devicewhich includes the light-emitting element having any of theabove-described structures and a means which controls the light-emittingelement.

Another embodiment of the present invention is a display device whichincludes the light-emitting element having any of the above-describedstructures in a display portion and a means which controls thelight-emitting element.

Another embodiment of the present invention is an electronic appliancewhich includes the light-emitting element having any of theabove-described structures.

The light-emitting device in this specification includes an imagedisplay device using a light-emitting element. Further, the category ofthe light-emitting device in this specification includes a module inwhich a light-emitting element is provided with a connector such as ananisotropic conductive film or a tape carrier package (TCP); a module inwhich the end of the TCP is provided with a printed wiring board; and amodule in which an IC (integrated circuit) is directly mounted on alight-emitting element by a COG (chip on glass) method. Furthermore, thecategory includes light-emitting devices which are used in lightingequipment or the like.

According to one embodiment of the present invention, a multicolorlight-emitting element in which light-emitting layers emitting light ofdifferent colors are stacked, in which color adjustment is easily madecan be provided.

According to another embodiment of the present invention, a multicolorlight-emitting element which is inexpensive and has favorable emissionefficiency can be provided.

According to another embodiment of the present invention, a multicolorlight-emitting element in which color adjustment is easily made andwhich is inexpensive and has favorable emission efficiency can beprovided.

According to another embodiment of the present invention, alight-emitting device, a display device, an electronic appliance, and alighting device each of which can be manufactured at low cost by usingany of the above-described light-emitting elements can be provided.

According to another embodiment of the present invention, alight-emitting device, a display device, an electronic appliance, and alighting device each of which has reduced power consumption by using anyof the above-described light-emitting elements can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram of a light-emitting element.

FIGS. 2A and 2B are conceptual diagrams of an active matrixlight-emitting device.

FIGS. 3A and 3B are conceptual diagrams of passive matrix light-emittingdevices.

FIG. 4 is a conceptual diagram of an active matrix light-emittingdevice.

FIGS. 5A and 5B are conceptual diagrams of an active matrixlight-emitting device.

FIGS. 6A and 6B are conceptual diagrams of a lighting device.

FIGS. 7A, 7B1, 7B2, 7C, and 7D illustrate electronic appliances.

FIG. 8 illustrates an electronic appliance.

FIG. 9 illustrates a lighting device.

FIG. 10 illustrates a lighting device.

FIG. 11 illustrates in-vehicle display devices and lighting devices.

FIGS. 12A to 12C illustrate an electronic appliance.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. Note that the present inventionis not limited to the following description, and it will be easilyunderstood by those skilled in the art that various changes andmodifications can be made without departing from the spirit and scope ofthe present invention. Therefore, the invention should not be construedas being limited to the description in the embodiments below.

Embodiment 1

As a multicolor light-emitting element in which light is obtained from aplurality of light-emitting substances, the following light-emittingelements have been proposed: a light-emitting element in which aplurality of emission center substances are contained in onelight-emitting layer, a light-emitting element in which a plurality oflight-emitting layers containing different emission center substancesare stacked, a light-emitting element in which an intermediate layer isprovided between light-emitting layers containing different emissioncenter substances, and the like.

It is known that in the light-emitting elements other than thelight-emitting element in which the intermediate layer is provided,energy transfer directly between the emission center substances orthrough a host material occurs and significantly affects emissionefficiency or an emission color.

The energy transfer is controlled by an element structure, selection ofa host material or an emission center substance, the presence or absenceof an additive substance, the amount of the additive substance, and thelike; however, adjusting them needs a lot of effort.

In addition, the element including the intermediate layer hasdisadvantages such as an increase in cost due to an increase in thenumber of layers to be formed and an increase in driving voltage.

In view of the above, in one embodiment of the present invention, amulticolor light-emitting element is provided in which a firstlight-emitting layer and a second light-emitting layer are stacked,light with different wavelengths is obtained from the first and secondlight-emitting layers, and light obtained by a combination of the lightwith different wavelengths is exhibited. The light obtained from thelight-emitting layers is obtained from exciplexes.

FIG. 1 is a schematic diagram of the light-emitting element of thisembodiment. In the light-emitting element of this embodiment, an ELlayer 103 is interposed between a first electrode 101 and a secondelectrode 102. One of the first electrode 101 and the second electrode102 functions as an anode and the other functions as a cathode. Notethat in FIG. 1, the first electrode 101 functions as an anode, and thesecond electrode 102 functions as a cathode.

The EL layer 103 includes at least a light-emitting layer 113. There isno particular limitation on the layers other than the light-emittinglayer 113 in the EL layer 103; for example, a hole-injection layer 111,a hole-transport layer 112, an electron-transport layer 114, and anelectron-injection layer 115 are included as illustrated in FIG. 1.

The light-emitting layer 113 includes a first light-emitting layer 113 aand a second light-emitting layer 113 b. The first light-emitting layer113 a contains at least a first organic compound and a second organiccompound. The second light-emitting layer 113 b contains at least athird organic compound and a fourth organic compound. Note that thefirst light-emitting layer 113 a may consist of only the first organiccompound and the second organic compound. In a similar manner, thesecond light-emitting layer 113 b may consist of only the third organiccompound and the fourth organic compound.

An exciplex here is an excited state formed from two kinds ofsubstances. In the case of photoexcitation, the exciplex is formed insuch a manner that one molecule in an excited state takes in the othersubstance in a ground state. Thus, when the exciplex emits light to bein a ground state, it returns to be the original substances. For thisreason, a ground state of the exciplex does not exist and energytransfer to the exciplex does not occur in principle. Thus, thelight-emitting element of this embodiment, in which energy transferbetween the light-emitting layers is suppressed, does not needcomplicated adjustment of the element structure for controlling energytransfer, so that desired light emission can be easily obtained fromboth of the light-emitting layers.

The exciplex is formed from two kinds of organic compounds as describedabove. Thus, the first light-emitting layer 113 a contains at least thefirst organic compound and the second organic compound, and the secondlight-emitting layer 113 b contains at least the third organic compoundand the fourth organic compound. In addition, the combination of thefirst organic compound and the second organic compound and thecombination of the third organic compound and the fourth organiccompound each form at least an exciplex.

A combination, in which one of the two kinds of organic compounds is acompound which easily accepts electrons (a material having anelectron-transport property) and the other is a compound which easilyaccepts holes (a material having a hole-transport property), ispreferable because the combination is advantageous for formation of anexciplex.

When one of the two kinds of organic compounds is the material having anelectron-transport property and the other is the material having ahole-transport property, the content ratio between the two kinds oforganic compounds in the light-emitting layer 113 is adjusted, wherebythe carrier balance in the light-emitting layer 113 can be easilyadjusted.

In the light-emitting element of this embodiment, a carrierrecombination region is formed in the vicinity of an interface betweenthe first light-emitting layer 113 a and the second light-emitting layer113 b, whereby excited energy can be distributed to the firstlight-emitting layer 113 a and the second light-emitting layer 113 b ina balanced manner; thus, light emission can be obtained from eachlight-emitting layer without difficulty. Further, when the combinationof the first organic compound and the second organic compound and thecombination of the third organic compound and the fourth organiccompound are each made to be a combination of the compound which easilyaccepts electrons (the material having an electron-transport property)and the compound which easily accepts holes (the material having ahole-transport property), by adjusting the mixture ratio, therecombination region can be easily adjusted so that it is formed at theinterface between the first light-emitting layer 113 a and the secondlight-emitting layer 113 b. Note that the intensity of emission fromeach light-emitting layer can be controlled by shifting the position ofthe recombination region; thus, the emission spectrum of thelight-emitting element can be easily adjusted. One of the firstlight-emitting layer 113 a and the second light-emitting layer 113 b,which is closer to the anode than the other light-emitting layer, may bea hole-transport layer, and the other light-emitting layer, which iscloser to the cathode than the one light-emitting layer, may be anelectron-transport layer in order to form the carrier recombinationregion in the vicinity of the interface between the first light-emittinglayer 113 a and the second light-emitting layer 113 b. Note that thehole-transport layer may contain a large amount of material having ahole-transport property and the electron-transport layer may contain alarge amount of material having an electron-transport property.

An exciplex exhibits light emission based on a difference in energybetween a shallower HOMO level (the absolute value thereof is smaller)of one of the two kinds of substances forming the exciplex and a deeperLUMO level (the absolute value thereof is larger) of the othersubstance. Thus, even when one of the first organic compound and thesecond organic compound is the same as one of the third organic compoundand the fourth organic compound, light with different wavelengths can beobtained from the first light-emitting layer and the secondlight-emitting layer. When one of the substances forming an exciplex inthe first light-emitting layer is the same as one of the substancesforming an exciplex in the second light-emitting layer, fewer kinds ofmaterials for forming the light-emitting element are used, so that thelight-emitting element can be manufactured more easily at lower cost.For this reason, the light-emitting element can be suitable for massproduction. Moreover, a carrier injection barrier at the interfacebetween the first light-emitting layer and the second light-emittinglayer can be lowered, which contributes to an increase in the lifetimeof the element.

Here, the excited states of an organic compound are a singlet excitedstate and a triplet excited state, and light from the singlet excitedstate (S1) is referred to as fluorescence, and light from the tripletexcited state (T1) is referred to as phosphorescence. The statisticalgeneration ratio of the excited states in the light-emitting element isconsidered to be S1:T1=1:3. Thus, a light-emitting element using aphosphorescent compound capable of converting the triplet excited stateinto light emission can have higher emission efficiency than alight-emitting element using a fluorescent compound. For this reason, alight-emitting element using a phosphorescent compound has been activelydeveloped recently.

However, most phosphorescent compounds currently available are complexescontaining a rare metal such as iridium as a central metal, which raisesconcern about the cost and the stability of supply.

As an emission mechanism capable of converting the triplet excitedenergy into light emission, there is delayed fluorescence besides theabove-described phosphorescence. The delayed fluorescence has amechanism in which the triplet excited state is upconverted into thesinglet excited state through reverse intersystem crossing, so thatlight emission is exhibited. The use of the delayed fluorescence makesit possible to obtain fluorescence with an internal quantum efficiencyexceeding 25%, which is considered to be the upper limit of the internalquantum efficiency of fluorescence.

The delayed fluorescence is likely to occur when the singlet excitedstate and the triplet excited state are close to each other. Since thesinglet excited state and the triplet excited state of an exciplex areclose to each other, delayed fluorescence is easily exhibited. The useof an exciplex which efficiently exhibits delayed fluorescence in thelight-emitting element of this embodiment can make the triplet excitedstate contribute to light emission, so that the light-emitting elementcan have high emission efficiency. Note that the delayed fluorescencehere includes what is called thermally activated delayed fluorescence(TADF) in which efficiency of reverse intersystem crossing is increasedby some heating (including self heat generation). In order that delayedfluorescence can be efficiently exhibited, the difference in energybetween the singlet excited state and the triplet excited state ispreferably greater than or equal to 0 eV and less than or equal to 0.2eV, more preferably greater than or equal to 0 eV and less than or equalto 0.1 eV.

Note that although an effect of an increase in emission efficiency canbe obtained as long as one of the light-emitting layers emits delayedfluorescence, it is more preferable that both of the light-emittinglayers emit delayed fluorescence.

In the case of a light-emitting element in which delayed fluorescence isexhibited, the external quantum efficiency might exceed 5% (singletexcited state generation rate 25%×light extraction efficiency 20%),which is said to be the theoretical limit of a fluorescent elementhardly exhibiting delayed fluorescence. When a light-emitting elementhaving the structure of the light-emitting element of this embodimenthas external quantum efficiency exceeding 5%, the light-emitting elementcan be assumed to exhibit delayed fluorescence efficiently.

In a different viewpoint, it can be said that delayed fluorescence isefficiently exhibited as long as the EL internal quantum efficiency Φe1(=Φp×25% (singlet excited state generation rate in EL)) estimated fromthe PL quantum efficiency Φp of the exciplex is lower than the internalquantum efficiency Φe2 (external quantum efficiency÷20% (lightextraction efficiency)) of the light-emitting element. Note that Φe2 ispreferably about twice as large as Φe1, in which case the use of thelight-emitting element of this embodiment is more effective.

In the light-emitting element of this embodiment having theabove-described structure, light with different wavelengths emitted fromthe first light-emitting layer and the second light-emitting layer isobtained from exciplexes, so that the light-emitting element can be amulticolor light-emitting element. The emission spectrum of such alight-emitting element has at least two peaks.

Further, although the first light-emitting layer and the secondlight-emitting layer are formed in contact with each other in thelight-emitting element of this embodiment, energy transfer between thelight-emitting layers is less likely to occur; thus, the balance betweenlight emission from the first light-emitting layer and light emissionfrom the second light-emitting layer can be easily adjusted.

For this reason, the light-emitting element can be suitably used as alight-emitting element exhibiting white light, in which control of anemission color is important. Thus, the light-emitting element can beeffectively used as a light-emitting element for lighting.

Since the exciplexes are used in the light-emitting layers in thelight-emitting element of this embodiment having the above-describedstructure, energy transfer between the light-emitting layers is lesslikely to occur, so that color adjustment of the light-emitting elementcan be easily made.

Moreover, in the light-emitting element of this embodiment, lightemission obtained from the exciplexes is utilized, so that delayedfluorescence is easily exhibited. The use of the delayed fluorescencecan convert the triplet excited energy into light emission; thus, thelight-emitting element can have high emission efficiency.

Furthermore, the exciplexes are used in the light-emitting element ofthis embodiment, so that energy transfer between the light-emittinglayers is less likely to occur and delayed fluorescence is easilyobtained. Thus, color adjustment of the light-emitting element can beeasily made, and the light-emitting element can have favorable emissionefficiency.

Embodiment 2

In this embodiment, a detailed example of the structure of thelight-emitting element described in Embodiment 1 is described below withreference to FIG. 1.

A light-emitting element in this embodiment includes, between a pair ofelectrodes, an EL layer including a plurality of layers. In thisembodiment, the light-emitting element includes the first electrode 101,the second electrode 102, and the EL layer 103 provided between thefirst electrode 101 and the second electrode 102. Note that in thisembodiment, the first electrode 101 functions as an anode and the secondelectrode 102 functions as a cathode. In other words, when voltage isapplied between the first electrode 101 and the second electrode 102 sothat the potential of the first electrode 101 is higher than that of thesecond electrode 102, light emission can be obtained.

Since the first electrode 101 functions as the anode, the firstelectrode 101 is preferably formed using any of metals, alloys,electrically conductive compounds with a high work function(specifically, a work function of 4.0 eV or more), mixtures thereof, andthe like. Specific examples are indium oxide-tin oxide (ITO: indium tinoxide), indium oxide-tin oxide containing silicon or silicon oxide,indium oxide-zinc oxide, indium oxide containing tungsten oxide and zincoxide (IWZO), and the like. Such conductive metal oxide films areusually formed by a sputtering method, but may also be formed byapplication of a sol-gel method or the like. In an example of theformation method, indium oxide-zinc oxide is deposited by a sputteringmethod using a target obtained by adding 1 wt % to 20 wt % of zinc oxideto indium oxide. Further, a film of indium oxide containing tungstenoxide and zinc oxide (IWZO) can be formed by a sputtering method using atarget in which tungsten oxide and zinc oxide are added to indium oxideat 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %, respectively. In addition,gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), anitride of a metal material (such as titanium nitride), or the like canbe used. Graphene can also be used. Note that when a composite materialdescribed later is used for a layer which is in contact with the firstelectrode 101 in the EL layer 103, an electrode material can be selectedregardless of its work function.

There is no particular limitation on the stacked structure of the ELlayer 103 as long as the light-emitting layer 113 has the structuredescribed in Embodiment 1. For example, the EL layer 103 can be formedby combining a hole-injection layer, a hole-transport layer, thelight-emitting layer, an electron-transport layer, an electron-injectionlayer, a carrier-blocking layer, an intermediate layer, and the like asappropriate. In this embodiment, the EL layer 103 has a structure inwhich a hole-injection layer 111, a hole-transport layer 112, alight-emitting layer 113, an electron-transport layer 114, and anelectron-injection layer 115 are stacked in this order over the firstelectrode 101. Materials for the layers are specifically given below.

The hole-injection layer 111 is a layer containing a substance having ahigh hole-injection property. Molybdenum oxide, vanadium oxide,ruthenium oxide, tungsten oxide, manganese oxide, or the like can beused. Alternatively, the hole-injection layer 111 can be formed using aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper phthalocyanine (abbreviation: CuPc), an aromatic aminecompound such as4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB) orN,N′-bis{4-[bis(3-methylphenyeamino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD), a high molecular compound such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS),or the like.

Alternatively, a composite material in which a material having ahole-transport property contains a material having an acceptor propertycan be used for the hole-injection layer 111. Note that the use of sucha substance having a hole-transport property which contains a substancehaving an acceptor property enables selection of a material used to forman electrode regardless of its work function. In other words, besides amaterial having a high work function, a material having a low workfunction can also be used for the first electrode 101. As the acceptorsubstance, 7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane(abbreviation: F₄-TCNQ), chloranil, and the like can be given. Inaddition, a transition metal oxide can be given. In addition, oxides ofmetals belonging to Group 4 to Group 8 of the periodic table can begiven. Specifically, vanadium oxide, niobium oxide, tantalum oxide,chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, andrhenium oxide are preferable because of their high electron-acceptingproperties. Among these, molybdenum oxide is especially preferablebecause it is stable in the air, has a low hygroscopic property, and iseasily handled.

As the material having a hole-transport property used for the compositematerial, any of a variety of organic compounds such as aromatic aminecompounds, carbazole derivatives, aromatic hydrocarbons, and highmolecular compounds (e.g., oligomers, dendrimers, or polymers) can beused. Note that the organic compound used for the composite material ispreferably an organic compound having a high hole-transport property.Specifically, a substance having a hole mobility of 10⁻⁶ cm²/Vs orhigher is preferably used. Organic compounds which can be used as thematerial having a hole-transport property in the composite material arespecifically given below.

Examples of the aromatic amine compound includeN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N′-bis[4-[bis(3-methylphenyl)amino]phenyl]-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

As carbazole derivatives which can be used for the composite material,the following can be given specifically:3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

In addition, examples of the carbazole derivatives which can be used forthe composite material include 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Examples of the aromatic hydrocarbon which can be used for the compositematerial include 2-tent-butyl-9,10-di(2-naphthyl)anthracene(abbreviation: t-BuDNA), 2-tent-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tent-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tent-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthypanthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, and 2,5,8,11-tetra(tert-butyl)perylene.Besides, pentacene, coronene, or the like can also be used. As thesearomatic hydrocarbons given here, it is preferable that an aromatichydrocarbon having a hole mobility of 1×10⁻⁶ cm²/Vs or more and having14 to 42 carbon atoms be used.

The aromatic hydrocarbon which can be used for the composite materialmay have a vinyl skeleton. Examples of the aromatic hydrocarbon having avinyl group include 4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation:DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation:DPVPA), and the like.

Moreover, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N-phenylamino}phenyl)methacrylamide](abbreviation:PTPDMA), or poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine(abbreviation: poly-TPD) can also be used.

By providing a hole-injection layer, a high hole-injection property canbe achieved to allow a light-emitting element to be driven at a lowvoltage.

The hole-transport layer 112 is a layer containing a material having ahole-transport property. Examples of the material having ahole-transport property include aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(Spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), and the like. The substances given here havehigh hole-transport properties and are mainly ones having a holemobility of 10⁻⁶ cm²/Vs or more. An organic compound given as an exampleof the material having a hole-transport property in the compositematerial described above can also be used for the hole-transport layer112. Moreover, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA)can also be used. Note that the layer containing a material having ahole-transport property is not limited to a single layer, and may be astack of two or more layers containing any of the above materials.

The light-emitting layer 113 has the structure of the light-emittinglayer 113, which is described in Embodiment 1. In other words, the firstlight-emitting layer 113 a and the second light-emitting layer 113 b arestacked in this order over the first electrode. The first light-emittinglayer 113 a contains a first organic compound and a second organiccompound. The second light-emitting layer 113 b contains a third organiccompound and a fourth organic compound. The light-emitting element ofthis embodiment is characterized in that the combination of the firstorganic compound and the second organic compound forms a first exciplexand the combination of the third organic compound and the fourth organiccompound forms a second exciplex. In addition, light emission isobtained from the first exciplex and the second exciplex.

There is no particular limitation on the materials which can be used asthe first organic compound, the second organic compound, the thirdorganic compound, and the fourth organic compound as long as thecombination of the materials satisfies the conditions described inEmbodiment 1. A variety of kinds of carrier-transport materials can beselected.

Examples of the material having an electron-transport property (materialwhich easily accepts electrons) include a heterocyclic compound having apolyazole skeleton, such asbis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); aheterocyclic compound having a polyazole skeleton such as2-(4-biphenylyl)-5-(4-tent-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyptris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), or2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); a heterocyclic compound having a diazineskeleton, such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine(abbreviation: 4,6mPnP2Pm), or4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); and a heterocyclic compound having a pyridine skeleton,such as 2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoline(abbreviation: 2mDBTBPDBQu-II),3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy), or1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB). Among theabove materials, a heterocyclic compound having a diazine skeleton and aheterocyclic compound having a pyridine skeleton have high reliabilityand are thus preferable. Specifically, a heterocyclic compound having adiazine (pyrimidine or pyrazine) skeleton has a high electron-transportproperty to contribute to a reduction in drive voltage.

Examples of the material having a hole-transport property (materialwhich easily accepts holes) include a compound having an aromatic amineskeleton, such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(Spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF), orN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); a compound having a carbazole skeleton, such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound havinga thiophene skeleton such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), or4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and a compound having a furan skeleton, suchas 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation:DBF3P-II) or4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, a compoundhaving an aromatic amine skeleton and a compound having a carbazoleskeleton are preferable because these compounds are highly reliable andhave high hole-transport properties to contribute to a reduction indrive voltage.

Carrier-transport materials can be selected from known substances aswell as from the carrier-transport materials given above. An exciplex tobe formed exhibits light emission originating from a difference inenergy between the shallower HOMO level of the HOMO levels of the twocompounds to be combined and the deeper LUMO level of the LUMO levels ofthe two compounds to be combined; thus, the combination of the firstorganic compound and the second organic compound and the combination ofthe third organic compound and the fourth organic compound with whichlight emission with a desired wavelength can be achieved is selected.Note that one of the first organic compound and the second organiccompound may be the same as one of the third organic compound and thefourth organic compound. In this case, fewer kinds of materials forforming the light-emitting element can be used, so that thelight-emitting element is advantageous in terms of cost.

Further, the combination of a material having an electron-transportproperty as one organic compound and a material having a hole-transportproperty as the other organic compound is advantageous for the formationof an exciplex. The transport property of the light-emitting layer canbe easily adjusted and a recombination region can be easily adjusted bychanging the contained amount of each compound. The ratio of thecontained amount of the material having an electron-transport propertyto contained amount of the material having an electron-transportproperty may be 1:9 to 9:1.

The light-emitting layer 113 having the above-described structure can beformed by co-evaporation by a vacuum evaporation method, or an inkjetmethod, a spin coating method, a dip coating method, or the like using amixed solution.

Note that although the structure in which the first light-emitting layer113 a is formed on the anode side and the second light-emitting layer113 b is formed on the cathode side is described in this embodiment, thestacking order may be reversed. In other words, the secondlight-emitting layer 113 b may be formed on the anode side and the firstlight-emitting layer 113 a may be formed on the cathode side.

The other structure and effect of the light-emitting layer 113 are thesame as those described in Embodiment 1. Embodiment 1 is to be referredto.

The electron-transport layer 114 is a layer containing a material havingan electron-transport property. Example of the electron-transport layer114 is a layer containing a metal complex having a quinoline skeleton ora benzoquinoline skeleton, such as tris(8-quinolinolato)aluminum(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), or the like. Alternatively, a metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂), orthe like can be used. Other than the metal complexes,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can be used. Thesubstances given here have high electron-transport properties and aremainly ones having an electron mobility of 10⁻⁶ cm²/Vs or more. Notethat any of the above-described host materials having electron-transportproperties may be used for the electron-transport layer 114.

The electron-transport layer 114 is not limited to a single layer andmay be a stack of two or more layers containing any of the substancesgiven above.

Further, a layer for controlling transport of electron carriers may beprovided between the electron-transport layer and the light-emittinglayer. This is a layer formed by addition of a small amount of asubstance having a high electron-trapping property to the aforementionedmaterials having a high electron-transport property, and the layer iscapable of adjusting carrier balance by retarding transport of electroncarriers. Such a structure is very effective in preventing a problem(such as a reduction in element lifetime) caused when electrons passthrough the light-emitting layer.

In addition, an electron-injection layer 115 may be provided in contactwith the second electrode 102 between the electron-transport layer 114and the second electrode 102. For the electron-injection layer 115, analkali metal, an alkaline earth metal, or a compound thereof, such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂), can be used. For example, a layer that is formed using asubstance having an electron-transport property and contains an alkalimetal, an alkaline earth metal, or a compound thereof can be used. Notethat a layer that is formed using a substance having anelectron-transport property and contains an alkali metal or an alkalineearth metal is preferably used as the electron-injection layer 115, inwhich case electron injection from the second electrode 102 isefficiently performed.

For the second electrode 102, any of metals, alloys, electricallyconductive compounds, and mixtures thereof which have a low workfunction (specifically, a work function of 3.8 eV or less) or the likecan be used. Specific examples of such a cathode material are elementsbelonging to Groups 1 and 2 of the periodic table, such as alkali metals(e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), andstrontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metalssuch as europium (Eu) and ytterbium (Yb), alloys thereof, and the like.However, when the electron-injection layer is provided between thesecond electrode 102 and the electron-transport layer, for the secondelectrode 102, any of a variety of conductive materials such as Al, Ag,ITO, or indium oxide-tin oxide containing silicon or silicon oxide canbe used regardless of the work function. Films of these electricallyconductive materials can be formed by a sputtering method, an inkjetmethod, a spin coating method, or the like.

Any of a variety of methods can be used to form the EL layer 103regardless whether it is a dry process or a wet process. For example, avacuum evaporation method, an ink-jet method, a spin coating method orthe like may be used. A different formation method may be employed foreach electrode or each layer.

In addition, the electrode may be formed by a wet method using a sol-gelmethod, or by a wet method using paste of a metal material.Alternatively, the electrode may be formed by a dry method such as asputtering method or a vacuum evaporation method.

In the light-emitting element having the above-described structure,current flows due to a potential difference between the first electrode101 and the second electrode 102, and holes and electrons recombine inthe light-emitting layer 113 which contains a substance with a highlight-emitting property, so that light is emitted. That is, alight-emitting region informed in the light-emitting layer 113.

Light emission is extracted out through one or both of the firstelectrode 101 and the second electrode 102. Therefore, one or both ofthe first electrode 101 and the second electrode 102 arelight-transmitting electrodes. In the case where only the firstelectrode 101 is a light-transmitting electrode, light emission isextracted through the first electrode 101. In the case where only thesecond electrode 102 is a light-transmitting electrode, light emissionis extracted through the second electrode 102. In the case where boththe first electrode 101 and the second electrode 102 arelight-transmitting electrodes, light emission is extracted through thefirst electrode 101 and the second electrode 102.

The structure of the layers provided between the first electrode 101 andthe second electrode 102 is not limited to the above-describedstructure. Preferably, a light-emitting region where holes and electronsrecombine is positioned away from the first electrode 101 and the secondelectrode 102 so that quenching due to the proximity of thelight-emitting region and a metal used for electrodes andcarrier-injection layers can be prevented.

Further, in order that transfer of energy from an exciton generated inthe light-emitting layer can be suppressed, preferably, thehole-transport layer and the electron-transport layer which are incontact with the light-emitting layer 113, particularly acarrier-transport layer in contact with a side closer to thelight-emitting region in the light-emitting layer 113, are formed usinga substance having a wider band gap than the exciplex included in thelight-emitting layer.

A light-emitting element in this embodiment is preferably fabricatedover a substrate of glass, plastic, or the like. As the way of stackinglayers over the substrate, layers may be sequentially stacked from thefirst electrode 101 side or sequentially stacked from the secondelectrode 102 side. In a light-emitting device, although onelight-emitting element may be formed over one substrate, a plurality oflight-emitting elements may be formed over one substrate. With aplurality of light-emitting elements as described above formed over onesubstrate, a lighting device in which elements are separated or apassive-matrix light-emitting device can be manufactured. Alight-emitting element may be formed over an electrode electricallyconnected to a thin film transistor (TFT), for example, which is formedover a substrate of glass, plastic, or the like, so that an activematrix light-emitting device in which the TFT controls the drive of thelight-emitting element can be manufactured. Note that there is noparticular limitation on the structure of the TFT, which may be astaggered TFT or an inverted staggered TFT. In addition, crystallinityof a semiconductor used for the TFT is not particularly limited either;an amorphous semiconductor or a crystalline semiconductor may be used.In addition, a driver circuit formed in a a T substrate may be formedwith an n-type TFT and a p-type TFT, or with either an n-type TFT or ap-type TFT.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, a light-emitting device including the light-emittingelement described in Embodiments 1 and 2 is described.

In this embodiment, the light-emitting device manufactured using thelight-emitting element described in Embodiments 1 and 2 is describedwith reference to FIGS. 2A and 2B. Note that FIG. 2A is a top viewillustrating the light-emitting device and FIG. 2B is a cross-sectionalview of FIG. 2A taken along lines A-B and C-D. The light-emitting deviceincludes a driver circuit portion (source line driver circuit) 601, apixel portion 602, and a driver circuit portion (gate line drivercircuit) 603, which are to control light emission of the light-emittingelement and illustrated with dotted lines. Moreover, a reference numeral604 denotes a sealing substrate; 605, a sealing material; and 607, aspace surrounded by the sealing material 605.

Note that a lead wiring 608 is a wiring for transmitting signals to beinput to the source line driver circuit 601 and the gate line drivercircuit 603 and for receiving a video signal, a clock signal, a startsignal, a reset signal, and the like from an FPC (flexible printedcircuit) 609 serving as an external input terminal. Although only theFPC is illustrated here, a printed wiring board (PWB) may be attached tothe FPC. The light-emitting device in the present specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.2B. The driver circuit portion and the pixel portion are formed over anelement substrate 610; the source line driver circuit 601, which is adriver circuit portion, and one of the pixels in the pixel portion 602are illustrated here.

In the source line driver circuit 601, a CMOS circuit is formed in whichan n-channel TFT 623 and a p-channel TFT 624 are combined. In addition,the driver circuit may be formed with any of a variety of circuits suchas a CMOS circuit, a PMOS circuit, or an NMOS circuit. Although adriver-integrated type in which the driver circuit is formed over thesubstrate is described in this embodiment, the present invention is notlimited to this type and the driver circuit can be formed outside thesubstrate.

The pixel portion 602 is formed with a plurality of pixels including aswitching TFT 611, a current controlling TFT 612, and a first electrode613 connected electrically with a drain of the current controlling TFT.An insulator 614 is formed to cover the end portions of the firstelectrode 613. Here, the insulator 614 is formed using a positive typephotosensitive acrylic resin film.

In order to improve the coverage, the insulator 614 is formed to have acurved surface with curvature at its upper or lower end portion. Forexample, in the case where positive photosensitive acrylic is used for amaterial of the insulator 614, only the upper end portion of theinsulator 614 preferably has a curved surface with a curvature radius(0.2 μm to 3 μm). As the insulator 614, either a negative photosensitiveresin or a positive photosensitive resin can be used.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. As a material used for the first electrode 613functioning as an anode, a material having a high work function ispreferably used. For example, a single-layer film of an ITO film, anindium tin oxide film containing silicon, an indium oxide filmcontaining zinc oxide at 2 wt % to 20 wt %, a titanium nitride film, achromium film, a tungsten film, a Zn film, a Pt film, or the like, astack of a titanium nitride film and a film containing aluminum as itsmain component, a stack of three layers of a titanium nitride film, afilm containing aluminum as its main component, and a titanium nitridefilm, or the like can be used. The stacked-layer structure enables lowwiring resistance, favorable ohmic contact, and a function as an anode.

In addition, the EL layer 616 is formed by any of a variety of methodssuch as an evaporation method using an evaporation mask, an inkjetmethod, and a spin coating method. The EL layer 616 has the structuredescribed in Embodiments 1 and 2. Further, for another material includedin the EL layer 616, any of low molecular-weight compounds and polymericcompounds (including oligomers and dendrimers) may be used.

As a material used for the second electrode 617, which is foimed overthe EL layer 616 and functions as a cathode, a material having a lowwork function (e.g., Al, Mg, Li, Ca, or an alloy or a compound thereof,such as MgAg, Mgln, or AlLi) is preferably used. In the case where lightgenerated in the EL layer 616 is transmitted through the secondelectrode 617, a stack of a thin metal film and a transparent conductivefilm (e.g., ITO, indium oxide containing zinc oxide at 2 wt % to 20 wt%, indium tin oxide containing silicon, or zinc oxide (ZnO)) ispreferably used for the second electrode 617.

Note that the light-emitting element is formed with the first electrode613, the EL layer 616, and the second electrode 617. The light-emittingelement has the structure described in Embodiment 1 or 2. In thelight-emitting device of this embodiment, the pixel portion, whichincludes a plurality of light-emitting elements, may include both thelight-emitting element described in Embodiment 1 or 2 and alight-emitting element having a different structure.

Further, the sealing substrate 604 is attached to the element substrate610 with the sealing material 605, so that the light-emitting element618 is provided in the space 607 surrounded by the element substrate610, the sealing substrate 604, and the sealing material 605. The space607 may be filled with filler, or may be filled with an inert gas (suchas nitrogen or argon), or the sealing material 605. It is preferablethat the sealing substrate be provided with a recessed portion and thedesiccant 625 be provided in the recessed portion, in which casedeterioration due to influence of moisture can be suppressed.

An epoxy-based resin or glass frit is preferably used for the sealingmaterial 605. It is preferable that such a material do not transmitmoisture or oxygen as much as possible. As the sealing substrate 604, aglass substrate, a quartz substrate, or a plastic substrate formed offiberglass reinforced plastic (FRP), polyvinyl fluoride (PVF),polyester, acrylic, or the like can be used.

As described above, the light-emitting device which uses thelight-emitting element described in Embodiment 1 or 2 can be obtained.

The light-emitting device in this embodiment is fabricated using thelight-emitting element described in Embodiment 1 or 2 and thus can havefavorable characteristics. Specifically, since the light-emittingelement described in Embodiment 1 or 2 has favorable emissionefficiency, the light-emitting device can have reduced powerconsumption. In addition, since the light-emitting element described inEmbodiment 1 or 2 is easily manufactured, the light-emitting device canbe provided at low cost.

FIGS. 3A and 3B each illustrate an example of a light-emitting device inwhich full color display is achieved by formation of a light-emittingelement exhibiting white light emission and with the use of coloringlayers (color filters) and the like. In FIG. 3A, a substrate 1001, abase insulating film 1002, a gate insulating film 1003, gate electrodes1006, 1007, and 1008, a first interlayer insulating film 1020, a secondinterlayer insulating film 1021, a peripheral portion 1042, a pixelportion 1040, a driver circuit portion 1041, first electrodes 1024W,1024R, 1024G and 1024B of light-emitting elements, a partition 1025, anEL layer 1028, a second electrode 1029 of the light-emitting elements, asealing substrate 1031, a sealing material 1032, and the like areillustrated.

In FIG. 3A, coloring layers (a red coloring layer 1034R, a greencoloring layer 1034G; and a blue coloring layer 1034B) are provided on atransparent base material 1033. A black layer (a black matrix) 1035 maybe additionally provided. The transparent base material 1033 providedwith the coloring layers and the black layer is positioned and fixed tothe substrate 1001. Note that the coloring layers and the black layerare covered with an overcoat layer 1036. In FIG. 3A, light emitted frompart of the light-emitting layer does not pass through the coloringlayers, while light emitted from the other part of the light-emittinglayer passes through the coloring layers. Since light which does notpass through the coloring layers is white and light which passes throughany one of the coloring layers is red, blue, or green, an image can bedisplayed using pixels of the four colors.

FIG. 3B illustrates an example in which the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) are provided between the gate insulating film 1003and the first interlayer insulating film 1020. As in the structure, thecoloring layers may be provided between the substrate 1001 and thesealing substrate 1031.

The above-described light-emitting device is a light-emitting devicehaving a structure in which light is extracted from the substrate 1001side where the TFTs are formed (a bottom emission structure), but may bea light-emitting device having a structure in which light is extractedfrom the sealing substrate 1031 side (a top emission structure). FIG. 4is a cross-sectional view of a light-emitting device having a topemission structure. In this case, a substrate which does not transmitlight can be used as the substrate 1001. The process up to the step offoiniing of a connection electrode which connects the TFT and the anodeof the light-emitting element is performed in a manner similar to thatof the light-emitting device having a bottom emission structure. Then, athird interlayer insulating film 1037 is formed to cover an electrode1022. This insulating film may have a planarization function. The thirdinterlayer insulating film 1037 can be formed using a material similarto that of the second interlayer insulating film, and can alternativelybe formed using any other known material.

The first electrodes 1024W, 1024R, 1024G and 1024B of the light-emittingelements each function as an anode here, but may function as a cathode.Further, in the case of a light-emitting device having a top emissionstructure as illustrated in FIG. 4, the first electrodes are preferablyreflective electrodes. The EL layer 1028 is formed to have a structuresimilar to the structure of the EL layer 103, which is described inEmbodiment 1 or 2, with which white light emission can be obtained.

In the case of a top emission structure as illustrated in FIG. 4,sealing can be performed with the sealing substrate 1031 on which thecoloring layers (the red coloring layer 1034R, the green coloring layer1034G, and the blue coloring layer 1034B) are provided. The sealingsubstrate 1031 may be provided with the black layer (the black matrix)1035 which is positioned between pixels. The coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) and the black layer (the black matrix) may becovered with the overcoat layer 1036. Note that a light-transmittingsubstrate is used as the sealing substrate 1031.

Further, although an example in which full color display is performedusing four colors of red, green, blue, and white is shown here, there isno particular limitation and full color display using three colors ofred, green, and blue may be performed.

The light-emitting device in this embodiment is manufactured using thelight-emitting element described in Embodiment 1 or 2 and thus can havefavorable characteristics. Specifically, since the light-emittingelement described in Embodiment 1 or 2 has favorable emissionefficiency, the light-emitting device can have reduced powerconsumption. In addition, since the light-emitting element described inEmbodiments 1 or 2 is easily manufactured, the light-emitting device canbe provided at low cost.

An active matrix light-emitting device is described above, whereas apassive matrix light-emitting device is described below. FIGS. 5A and 5Billustrate a passive matrix light-emitting device manufactured using thepresent invention. FIG. 5A is a perspective view of the light-emittingdevice, and FIG. 5B is a cross-sectional view of FIG. 5A taken alongline X-Y. In FIGS. 5A and 5B, an EL layer 955 is provided between anelectrode 952 and an electrode 956 over a substrate 951. An end portionof the electrode 952 is covered with an insulating layer 953. Apartition layer 954 is provided over the insulating layer 953. Thesidewalls of the partition layer 954 are aslope such that the distancebetween both sidewalls is gradually narrowed toward the surface of thesubstrate. In other words, a cross section taken along the direction ofthe short side of the partition wall layer 954 is trapezoidal, and thelower side (a side which is in the same direction as a plane directionof the insulating layer 953 and in contact with the insulating layer953) is shorter than the upper side (a side which is in the samedirection as the plane direction of the insulating layer 953 and not incontact with the insulating layer 953. The partition layer 954 thusprovided can prevent defects in the light-emitting element due to staticelectricity or the like. Further, also in the passive matrixlight-emitting device, the light-emitting element described inEmbodiment 1 or 2, which has favorable emission efficiency, is used, sothat the light-emitting device can have less power consumption.Moreover, since the light-emitting element described in Embodiment 1 or2 is easily manufactured, the light-emitting device can be provided atlow cost.

Since many minute light-emitting elements arranged in a matrix in thelight-emitting device described above can each be controlled, thelight-emitting device can be suitably used as a display device fordisplaying images.

This embodiment can be freely combined with any of other embodiments.

Embodiment 4

In this embodiment, an example in which the light-emitting elementdescribed in Embodiment 1 or 2 is used for a lighting device isdescribed with reference to FIGS. 6A and 6B. FIG. 6B is a top view ofthe lighting device, and FIG. 6A is a cross-sectional view of FIG. 6Ataken along line e-f.

In the lighting device in this embodiment, a first electrode 401 isformed over a substrate 400 which is a support and has alight-transmitting property. The first electrode 401 corresponds to thefirst electrode 101 in Embodiment 1. When light is extracted through thefirst electrode 401 side, the first electrode 401 is formed using amaterial having a light-transmitting property.

A pad 412 for applying voltage to a second electrode 404 is providedover the substrate 400.

An EL layer 403 is formed over the first electrode 401. The structure ofthe EL layer 403 corresponds to, for example, the structure of the ELlayer 103 in Embodiment 1. For these structures, the description inEmbodiment 1 can be referred to.

The second electrode 404 is fon led to cover the EL layer 403. Thesecond electrode 404 corresponds to the second electrode 102 inEmbodiment 1. The second electrode 404 is formed using a material havinghigh reflectance when light is extracted through the first electrode 401side. The second electrode 404 is connected to the pad 412, wherebyvoltage is applied thereto.

As described above, the lighting device described in this embodimentincludes a light-emitting element including the first electrode 401, theEL layer 403, and the second electrode 404. Since the light-emittingelement has high emission efficiency, the lighting device in thisembodiment can be a lighting device having low power consumption. Inaddition, since the light-emitting element described in Embodiment 1 and2 is easily manufactured, the lighting device can be provided at lowcost.

The light-emitting element having the above structure is fixed to asealing substrate 407 with sealing materials 405 and 406 and sealing isperformed, whereby the lighting device is completed. It is possible touse only either the sealing material 405 or the sealing material 406. Inaddition, the inner sealing material 406 (not shown in FIG. 6B) can bemixed with a desiccant which enables moisture to be adsorbed, increasingreliability.

When parts of the pad 412 and the first electrode 401 are extended tothe outside of the sealing materials 405 and 406, the extended parts canserve as external input terminals. An IC chip 420 mounted with aconverter or the like may be provided over the external input terminals.

As described above, since the lighting device described in thisembodiment includes the light-emitting element described in Embodiment 1or 2 as an EL element, the lighting device can be a lighting devicehaving low power consumption. Further, the lighting device can be alighting device driven at low voltage. Moreover, since thelight-emitting element described in Embodiment 1 or 2 is easilymanufactured, the light-emitting device can be provided at low cost.

Embodiment 5

In this embodiment, examples of electronic appliances each including thelight-emitting element described in Embodiment 1 or 2 are described. Thelight-emitting element described in Embodiment 1 or 2 has favorableemission efficiency and reduced power consumption. As a result, theelectronic appliances described in this embodiment can each include alight-emitting portion having reduced power consumption. In addition,since the light-emitting element described in Embodiment 1 or 2 iseasily manufactured, the electronic appliance can be provided at lowcost.

Examples of the electronic appliance to which the above light-emittingelement is applied include television devices (also referred to as TV ortelevision receivers), monitors for computers and the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cell phones or mobile phone devices),portable game machines, portable information terminals, audio playbackdevices, large game machines such as pachinko machines, and the like.Specific examples of these electronic appliances are described below.

FIG. 7A illustrates an example of a television device. In the televisiondevice, a display portion 7103 is incorporated in a housing 7101. Here,the housing 7101 is supported by a stand 7105. Images can be displayedon the display portion 7103, and in the display portion 7103, thelight-emitting elements described in Embodiment 1 and 2 are arranged ina matrix. The light-emitting elements can have favorable emissionefficiency. Further, the light-emitting elements can be driven at lowvoltage. Moreover, the light-emitting elements can have a long lifetime.Therefore, the television device including the display portion 7103which is formed using the light-emitting element can have reduced powerconsumption. Further, the television device can be driven at lowvoltage. Moreover, the television device can be inexpensive.

The television device can be operated with an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Further, the remote controller 7110 may be provided with adisplay portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 7B1 illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured using light-emitting elements arranged ina matrix in the display portion 7203, which are the same as thatdescribed in Embodiment 1 or 2. The computer illustrated in FIG. 7B1 mayhave a structure illustrated in FIG. 7B2. The computer illustrated inFIG. 7B2 is provided with a second display portion 7210 instead of thekeyboard 7204 and the pointing device 7206. The second display portion7210 is a touch screen, and input can be performed by operation ofdisplay for input on the second display portion 7210 with a finger or adedicated pen. The second display portion 7210 can also display imagesother than the display for input. The display portion 7203 may also be atouchscreen. Connecting the two screens with a hinge can preventtroubles; for example, the screens can be prevented from being crackedor broken while the computer is being stored or carried. Note that thiscomputer is manufactured using light-emitting elements arranged in amatrix in the display portion 7203, which are the same as that describedin Embodiment 1 and 2. The light-emitting elements can have favorableemission efficiency. Therefore, this computer having the display portion7203 which is formed using the light-emitting elements consumes lesspower. In addition, the computer can be inexpensive.

FIG. 7C illustrates a portable game machine, which includes twohousings, a housing 7301 and a housing 7302, which are connected with ajoint portion 7303 so that the portable game machine can be opened orfolded. The housing 7301 incorporates a display portion 7304 includingthe light-emitting elements each of which is described in Embodiment 1and 2 and which are arranged in a matrix, and the housing 7302incorporates a display portion 7305. In addition, the portable gamemachine illustrated in FIG. 7C includes a speaker portion 7306, arecording medium insertion portion 7307, an LED lamp 7308, an inputmeans (an operation key 7309, a connection terminal 7310, a sensor 7311(a sensor having a function of measuring force, displacement, position,speed, acceleration, angular velocity, rotational frequency, distance,light, liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), anda microphone 7312), and the like. Needless to say, the structure of theportable game machine is not limited to the above as long as the displayportion including the light-emitting elements each of which is describedin Embodiment 1 and 2 and which are arranged in a matrix is used as atleast either the display portion 7304 or the display portion 7305, orboth, and the structure can include other accessories as appropriate.The portable game machine illustrated in FIG. 7C has a function ofreading out a program or data stored in a storage medium to display iton the display portion, and a function of sharing information withanother portable game machine by wireless communication. The portablegame machine illustrated in FIG. 7C can have a variety of functionswithout limitation to the above. The portable game machine having thedisplay portion 7304 can consume less power because the light-emittingelements used in the display portion 7304 have favorable emissionefficiency. Since the light-emitting elements used in the displayportion 7304 has low driving voltage, the portable game machine can alsobe a portable game machine having low driving voltage. In addition,since the light-emitting element described in Embodiment 1 or 2 iseasily manufactured, the portable game machine can be provided at lowcost.

FIG. 7D illustrates an example of a mobile phone. The mobile phone isprovided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400has the display portion 7402 including the light-emitting elements eachof which is described in Embodiment 1 and 2 and which are arranged in amatrix. The light-emitting elements can have favorable emissionefficiency. In addition, the light-emitting element can have low drivingvoltage. Furthermore, the light-emitting element can have a longlifetime. Therefore, this mobile phone having the display portion 7402which is formed using the light-emitting elements consumes less power.In addition, the mobile phone can have low driving voltage. Moreover,since the light-emitting element described in Embodiment 1 or 2 iseasily manufactured, the mobile phone can be provided at low cost.

When the display portion 7402 of the mobile phone illustrated in FIG. 7Dis touched with a finger or the like, data can be input into the mobilephone. In this case, operations such as making a call and creating ane-mail can be performed by touch on the display portion 7402 with afinger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing an e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be inputted. In this case,it is preferable to display a keyboard or number buttons on almost theentire screen of the display portion 7402.

When a detection device which includes a sensor for detectinginclination, such as a gyroscope or an acceleration sensor, is providedinside the mobile phone, the direction of the cellular phone (whetherthe cellular phone is placed horizontally or vertically for a landscapemode or a portrait mode) is determined so that display on the screen ofthe display portion 7402 can be automatically switched.

The screen modes are switched by touching the display portion 7402 oroperating the operation buttons 7403 of the housing 7401. Alternatively,the screen modes can be switched depending on kinds of images displayedon the display portion 7402. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is switched to the display mode. When the signal is a signalof text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed within a specified period while a signal detectedby an optical sensor in the display portion 7402 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

Note that the structure described in this embodiment can be combinedwith any of the structures described in Embodiments 1 to 4 asappropriate.

As described above, the application range of the light-emitting devicehaving the light-emitting element described in Embodiment 1 and 2 iswide so that this light-emitting device can be applied to electronicappliances in a variety of fields. By using the light-emitting elementdescribed in Embodiment 1 and 2, an electronic appliance having reducedpower consumption can be obtained. In addition, the light-emittingelement described in Embodiment 1 or 2 can be easily manufactured, theelectronic appliance can be provided at low cost.

FIG. 8 illustrates an example of a liquid crystal display device usingthe light-emitting element described in Embodiment 1 and 2 for abacklight. The liquid crystal display device shown in FIG. 8 includes ahousing 901, a liquid crystal layer 902, a backlight unit 903, and ahousing 904. The liquid crystal layer 902 is connected to a driver IC905. The light-emitting element described in Embodiment 1 and 2 is usedfor the backlight unit 903, to which current is supplied through aterminal 906.

The light-emitting element described in Embodiment 1 and 2 is used forthe backlight of the liquid crystal display device; thus, the backlightcan have reduced power consumption. In addition, the use of thelight-emitting element described in Embodiment 2 enables manufacture ofa planar-emission lighting device and further a larger-areaplanar-emission lighting device; therefore, the backlight can be alarger-area backlight, and the liquid crystal display device can also bea larger-area device. Furthermore, the light-emitting device using thelight-emitting element described in Embodiment 2 can be thinner than aconventional one; accordingly, the display device can also be thinner.

FIG. 9 illustrates an example in which the light-emitting elementdescribed in Embodiment 1 and 2 is used for a table lamp which is alighting device. The table lamp illustrated in FIG. 9 includes a housing2001 and a light source 2002, and the lighting device described inEmbodiment 4 is used for the light source 2002.

FIG. 10 illustrates an example in which the light-emitting elementdescribed in Embodiment 1 and 2 is used for an indoor lighting device3001. Since the light-emitting element described in Embodiment 1 and 2has reduced power consumption, a lighting device having reduced powerconsumption can be obtained. Further, since the light-emitting elementdescribed in Embodiment 1 and 2 can have a large area, thelight-emitting element can be used for a large-area lighting device.Furthermore, since the light-emitting element described in Embodiment 1and 2 is thin, the light-emitting element can be used for a lightingdevice having a reduced thickness.

The light-emitting element described in Embodiment 1 and 2 can also beused for an automobile windshield or an automobile dashboard. FIG. 11illustrates one mode in which the light-emitting element described inEmbodiment 2 is used for an automobile windshield and an automobiledashboard. Displays 5000 to 5005 each include the light-emittingelements described in Embodiments 1 and 2.

The display 5000 and the display 5001 are provided in the automobilewindshield in which the light-emitting elements described in Embodiment1 and 2 are incorporated. The light-emitting element described inEmbodiment 1 and 2 can be formed into what is called a see-throughdisplay device, through which the opposite side can be seen, byincluding a first electrode and a second electrode formed of electrodeshaving light-transmitting properties. Such see-through display devicescan be provided even in the automobile windshield, without hindering thevision. Note that in the case where a transistor for driving or the likeis provided, a transistor having a light-transmitting property, such asan organic transistor using an organic semiconductor material or atransistor using an oxide semiconductor, is preferably used.

A display device incorporating the light-emitting element described inEmbodiment 1 and 2 is provided in the display 5002 in a pillar portion.The display 5002 can compensate for the view hindered by the pillarportion by showing an image taken by an imaging unit provided in the carbody. Similarly, the display 5003 provided in the dashboard cancompensate for the view hindered by the car body by showing an imagetaken by an imaging unit provided in the outside of the car body, whichleads to elimination of blind areas and enhancement of safety. Showingan image so as to compensate for the area which a driver cannot seemakes it possible for the driver to confirm safety easily andcomfortably.

The display 5004 and the display 5005 can provide a variety of kinds ofinfo illation such as navigation data, a speedometer, a tachometer, amileage, a fuel meter, a gearshift indicator, and air-condition setting.The content or layout of the display can be changed freely by a user asappropriate. Further, such information can also be shown by the displays5000 to 5003. Note that the displays 5000 to 5005 can also be used aslighting devices.

The light-emitting element described in Embodiment 1 and 2 can have highemission efficiency and low power consumption. Therefore, load on abattery is small even when a number of large screens such as thedisplays 5000 to 5005 are provided, which provides comfortable use. Forthat reason, the light-emitting device and the lighting device each ofwhich includes the light-emitting element described in Embodiment 1 and2 can be suitably used as an in-vehicle light-emitting device and anin-vehicle lighting device.

FIGS. 12A and 12B illustrate an example of a foldable tablet terminal.The tablet terminal is opened in FIG. 12A. The tablet terminal includesa housing 9630, a display portion 9631 a, a display portion 9631 b, adisplay mode switch 9034, a power switch 9035, a power saver switch9036, a clasp 9033, and an operation switch 9038. Note that in thetablet terminal, one or both of the display portion 9631 a and thedisplay portion 9631 b is/are formed using a light-emitting device whichincludes the light-emitting element described in Embodiment 1 and 2.

Part of the display portion 9631 a can be a touchscreen region 9632 aand data can be input when a displayed operation key 9637 is touched.Although half of the display portion 9631 a has only a display functionand the other half has a touchscreen function, one embodiment of thepresent invention is not limited to the structure. The whole displayportion 9631 a may have a touchscreen function. For example, a keyboardcan be displayed on the entire region of the display portion 9631 a sothat the display portion 9631 a is used as a touchscreen, and thedisplay portion 9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touchscreen region 9632 b. A switching button 9639 forshowing/hiding a keyboard of the touch panel is touched with a finger, astylus, or the like, so that keyboard buttons can be displayed on thedisplay portion 9631 b.

Touch input can be performed in the touchscreen region 9632 a and thetouchscreen region 9632 b at the same time.

The display mode switch 9034 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. With the switch 9036 for switching topower-saving mode, the luminance of display can be optimized inaccordance with the amount of external light at the time when the tabletis in use, which is detected with an optical sensor incorporated in thetablet. The tablet may include another detection device such as a sensorfor detecting orientation (e.g., a gyroscope or an acceleration sensor)in addition to the optical sensor.

Although FIG. 12A illustrates an example in which the display portion9631 a and the display portion 963 b have the same display area, oneembodiment of the present invention is not limited to the example. Thedisplay portion 9631 a and the display portion 9631 b may have differentdisplay areas and different display quality. For example, one of themmay be a display panel that can display higher-definition images thanthe other.

The tablet terminal is folded in FIG. 12B. The tablet terminal includesthe housing 9630, a solar cell 9633, a charge and discharge controlcircuit 9634, a battery 9635, and a DC-to-DC converter 9636. Note thatFIG. 12B illustrates an example in which the charge and dischargecontrol circuit 9634 includes the battery 9635 and the DC-to-DCconverter 9636.

Since the tablet terminal can be folded, the housing 9630 can be closedwhen not in use. Thus, the display portions 9631 a and 9631 b can beprotected, thereby providing a tablet terminal with high endurance andhigh reliability for long-term use.

In addition, the tablet terminal illustrated in FIGS. 12A and 12B canhave a function of displaying various kinds of information (e.g., astill image, a moving image, and a text image) on the display portion, afunction of displaying a calendar, the date, the time, or the like onthe display portion, a touch input function of operating or editinginformation displayed on the display portion by touch input, a functionof controlling processing by various kinds of software (programs), andthe like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 9633is preferably provided on one or two surfaces of the housing 9630, inwhich case the battery 9635 can be charged efficiently.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 12B are described with reference to a blockdiagram of FIG. 12C. FIG. 12C shows the solar cell 9633, the battery9635, the DC-to-DC converter 9636, a converter 9638, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DC-to-DCconverter 9636, the converter 9638, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 in FIG. 12B.

First, an example of operation in the case where power is generated bythe solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell is raised or lowered by the DC-to-DCconverter 9636 so that the power has voltage for charging the battery9635. Then, when power supplied from the battery 9635 charged by thesolar cell 9633 is used for the operation of the display portion 9631,the switch SW1 is turned on and the voltage of the power is raised orlowered by the converter 9638 so as to be voltage needed for the displayportion 9631. In addition, when display on the display portion 9631 isnot performed, the switch SW1 is turned off and a switch SW2 is turnedon so that charge of the battery 9635 may be performed.

Although the solar cell 9633 is described as an example of a powergeneration means, the power generation means is not particularlylimited, and the battery 9635 may be charged by another power generationmeans such as a piezoelectric element or a thermoelectric conversionelement (Peltier element). The battery 9635 may be charged by anon-contact power transmission module which is capable of charging bytransmitting and receiving power by wireless (without contact), oranother charge means used in combination, and the power generation meansis not necessarily provided.

One embodiment of the present invention is not limited to the tabletterminal having the shape illustrated in FIGS. 12A to 12C as long as thedisplay portion 9631 is included.

This application is based on Japanese Patent Application Ser. No.2012-172937 filed with Japan Patent Office on Aug. 3, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. (canceled)
 2. A light-emitting elementcomprising: a first electrode; a second electrode; and a firstlight-emitting layer over the first electrode, the first light-emittinglayer comprising a first organic compound and a second organic compound,a second light-emitting layer over the first light-emitting layercomprising a third organic compound and a fourth organic compound,wherein the first organic compound and the second organic compound aremixed in the first light-emitting layer, wherein the third organiccompound and the fourth organic compound are mixed in the secondlight-emitting layer, wherein the second organic compound is capable offouling a first exciplex with the first organic compound, wherein thefourth organic compound is capable of for a second exciplex with thefirst organic compound, wherein the first exciplex has a first emissionpeak, wherein the second exciplex has a second emission peak, andwherein the first emission peak and the second emission peak aredifferent from each other.
 3. The light-emitting element according toclaim 2, wherein light emission is obtained from the first exciplex andthe second exciplex.
 4. The light-emitting element according to claim 2,wherein a difference in energy between a singlet excited level and atriplet excited level of at least one of the first exciplex and thesecond exciplex is 0.2 eV or lower.
 5. The light-emitting elementaccording to claim 2, wherein at least one of the first exciplex and thesecond exciplex exhibits delayed fluorescence.
 6. The light-emittingelement according to claim 2, wherein the first organic compound is amaterial having an electron-transport property, wherein the secondorganic compound is a material having a hole-transport property, whereinthe third organic compound is a material having an electron-transportproperty, and wherein the fourth organic compound is a material having ahole-transport property.
 7. The light-emitting element according toclaim 6, wherein the first electrode functions as an anode, and thesecond electrode functions as a cathode, wherein the firstlight-emitting layer comprises a larger amount of the material havingthe hole-transport property than the second light-emitting layer, andwherein the second light-emitting layer comprises a larger amount of thematerial having the electron-transport property than the firstlight-emitting layer.
 8. The light-emitting element according to claim2, wherein an emission spectrum of the light-emitting element has twopeaks.
 9. The light-emitting element according to claim 2, wherein oneof the first organic compound and the second organic compound is thesame as one of the third organic compound and the fourth organiccompound.
 10. The light-emitting element according to claim 2, whereinthe light-emitting layer exhibits white light emission.
 11. Alight-emitting device comprising: the light-emitting element accordingto claim 2; and a flexible substrate.
 12. A lighting device comprisingthe light-emitting element according to claim
 2. 13. An electronicappliance comprising the light-emitting element according to claim 2,wherein the electronic appliance is one of television devices, monitorsfor computers, digital cameras and digital video cameras, digital photoframes, mobile phones, portable game machines, portable informationterminals, audio playback devices and game machines.
 14. Alight-emitting element comprising: a first electrode; a secondelectrode; and a first light-emitting layer over the first electrode,the first light-emitting layer comprising a first organic compound and asecond organic compound, a second light-emitting layer over the firstlight-emitting layer comprising a third organic compound and a fourthorganic compound, wherein the first organic compound and the secondorganic compound are mixed in the first light-emitting layer, whereinthe third organic compound and the fourth organic compound are mixed inthe second light-emitting layer, wherein the second organic compound iscapable of forming a first exciplex with the first organic compound,wherein the fourth organic compound is capable of forming a secondexciplex with the first organic compound, wherein the first exciplex hasa first emission peak, wherein the second exciplex has a second emissionpeak, wherein the first emission peak and the second emission peak aredifferent from each other, and wherein emission of the light-emittingelement includes emission of the first exciplex and emission the secondexciplex.
 15. The light-emitting element according to claim 14, whereinone of the first organic compound and the second organic compound is thesame as one of the third organic compound and the fourth organiccompound.
 16. The light-emitting element according to claim 14, whereinthe light-emitting layer exhibits white light emission.
 17. Thelight-emitting element according to claim 14, wherein the first organiccompound is a material having an electron-transport property, whereinthe second organic compound is a material having a hole-transportproperty, wherein the third organic compound is a material having anelectron-transport property, wherein the fourth organic compound is amaterial having a hole-transport property. wherein an amount ratio ofthe second organic compound to the first organic compound in the firstlight-emitting layer is larger than an amount ratio of the fourthorganic compound to the third organic compound in the secondlight-emitting layer, and wherein an amount ratio of the first organiccompound to the second organic compound in the first light-emittinglayer is larger than an amount ratio of the third organic compound tothe fourth organic compound in the second light-emitting layer.
 18. Alight-emitting device comprising: the light-emitting element accordingto claim 14; and a flexible substrate.
 19. A lighting device comprisingthe light-emitting element according to claim
 14. 20. An electronicappliance comprising the light-emitting element according to claim 14,wherein the electronic appliance is one of television devices, monitorsfor computers, digital cameras and digital video cameras, digital photoframes, mobile phones, portable game machines, portable informationterminals, audio playback devices and game machines.